Tandem master cylinder for electrohydraulic braking systems

ABSTRACT

This master cylinder comprises a body ( 12 ) defining a primary pressure chamber ( 14 ) and a secondary pressure chamber ( 16 ), separated by an axially movable piston ( 22 ), the so-called secondary piston; means ( 28 ) for the hydraulic connection of the primary pressure chamber ( 14 ) with braking-force simulation means ( 30 ); and complementary moving and stationary means ( 32, 34 ) for the sealing of the hydraulic connection means ( 28 ). The moving and stationary sealing means ( 32, 34 ) are borne by the secondary piston ( 22 ) and the body ( 12 ) respectively. The secondary piston ( 22 ) is axially movable between a rest position, in which the complementary sealing means ( 32, 34 ) are spaced apart from each other, and a position in which said complementary sealing means ( 32, 34 ) are cooperating with each other. The secondary piston ( 22 ) comprises two portions ( 22 P,  22 D), capable of an axial motion in relation to each other in order to adjust the relative axial position of the moving and stationary sealing means ( 32, 34 ) when the secondary piston ( 22 ) is in its rest position.

The present invention relates to a tandem master cylinder intended foran electrohydraulic braking system.

A tandem master cylinder intended for an electrohydraulic braking systemis already well known from the state of the art, such device being ofthe type comprising:

-   -   a body, defining primary and secondary pressure chambers,        separated by an axially movable piston, the so-called secondary        piston;    -   means for the hydraulic connection of the primary pressure        chamber with braking-force simulation means; and    -   complementary moving and stationary means for the sealing of the        hydraulic connection means, and borne by the secondary piston        and the body respectively, the secondary piston being axially        movable between a rest position, in which the moving and        stationary sealing means are spaced apart from each other, and a        position in which these moving and stationary sealing means are        cooperating with each other.

Usually, the driver depresses a brake pedal, connected to the secondarypiston, so as to control the braking of the vehicle and the travel ofthe brake pedal is transmitted to a hydraulic braking circuit throughvarious means.

A braking system comprising a master cylinder of the above-mentionedtype is capable of operating in both normal and emergencyconfigurations.

When the braking system is in a normal braking mode, the hydraulicbraking circuit is isolated from the master cylinder and the travel ofthe brake pedal is transmitted to the hydraulic braking circuit throughelectrical means.

On the other hand, when the braking system is in the emergency brakingmode, the hydraulic braking circuit is connected with the mastercylinder and the travel of the brake pedal is transmitted to thehydraulic braking circuit through the brake fluid, contained inside saidmaster cylinder.

Usually, the stationary sealing means comprise an annular bearing seat,provided in the body, and the moving sealing means comprise a O-ringborne by the secondary piston and intended for a cooperation with suchbearing seat.

In the normal braking configuration of the braking system, when thedriver depresses the brake pedal, such action results in a travel of thesecondary piston through a short stroke and consequently the moving andstationary sealing means may remain spaced apart from each other and,therefore, the hydraulic connection means may remain in the open state.

In such a normal operating configuration, the simulation means give thedriver, as he depresses the brake pedal, a braking feeling like that hewould get if the master cylinder were connected with the hydraulicbraking circuit.

In the emergency operating configuration of the braking system, theforce exerted by the driver on the brake pedal results in a travel ofthe secondary piston through a long stroke, at the end of which themoving and stationary sealing means are cooperating with each other, soas to close the hydraulic means connecting the primary pressure chamberwith the braking-force simulation means.

During the long stroke of the piston, owing to the fact that thehydraulic connection means are not closed yet, some brake fluid istransferred from the primary pressure chamber into the simulation means.As a matter of fact, the smaller the quantity of brake fluid transferredto the simulation means, the faster the pressure rise in the primarypressure chamber, which means a more efficient braking operation.

The object of this invention is to limit as much as possible the strokeof the secondary piston when the driver depresses the brake pedal in anemergency braking configuration, so as to limit the quantity of brakefluid, which is transferred from the primary pressure chamber towardsthe braking-force simulation means.

To this end, the invention deals with a tandem master cylinder for anelectrohydraulic braking system, of the above-mentioned type andcharacterised in that the secondary piston comprises two portions,capable of an axial motion in relation to each other in order to adjustthe relative axial position of the moving and stationary sealing meanswhen the secondary piston is in its rest position.

According to features of various embodiments of said master cylinder:

-   -   these two portions of the secondary piston comprise male and        female elements respectively, connecting up these portions of        the secondary piston and provided with complementary threaded        surfaces, in such a way that the male element constitutes a        screw and the female element forms a nut;    -   the master cylinder comprises means for the locking of the male        and female elements in a predetermined relative position;    -   the locking means comprise a check nut, screwed on the male        element and resting on the female element;    -   the locking means comprise at least a radial deformation of one        of the threaded surfaces of the male and female elements;    -   the stationary sealing means comprise an annular bearing seat        provided in the body, and the moving sealing means comprise a        O-ring, borne by the secondary piston and intended for a        cooperation with said bearing seat;    -   a first portion of the secondary piston is intended for a        cooperation with a rest stop, rigidly linked with the body,        while the second portion of the secondary piston bears the        moving sealing means;    -   the rest stop comprises a pin, which is rigidly linked with the        body and which extends substantially transversely to the travel        direction of the secondary piston, such pin being accommodated        in an elongate cavity provided in the first portion of said        piston;    -   the elongate cavity communicates with a brake-fluid supply        chamber, called the secondary supply chamber, the first portion        of the secondary piston being provided with a passage connecting        up the secondary supply and pressure chambers, and closable by a        valve;    -   the valve is fitted with a control stem, extending through the        passage provided in the secondary piston, and intended for a        cooperation with the bearing pin so as to keep the valve in the        open position.

Other features and advantages of the present invention will be apparentfrom the following detailed description, by way of example and by nomeans as a limitation, when taken in conjunction with the accompanyingdrawings, in which FIGS. 1 and 2 are partial axial sectional views of amaster cylinder for a braking system according to a first and a secondembodiment of this invention, respectively.

FIG. 1 shows a tandem master cylinder for an electrohydraulic brakingsystem according to a first embodiment of the invention, said mastercylinder being generally referred to by 10 as a whole. In the describedexample, the tandem master cylinder 10 is of the valved type.

As a rule and throughout the following description, an element or aportion is called “proximal” when it is near the brake pedal actuated bythe driver, taking into consideration the kinematic chain connectingsaid element or portion to the brake pedal, and it is called “distal” inthe contrary case.

The master cylinder 10 comprises a body 12 defining two brake-fluidpressurizing chambers, which are usually called the primary pressurechamber 14 and the secondary pressure chamber 16.

The body 12 also defines a supply chamber 18 for the secondary pressurechamber 16. Such chamber 18, usually called the secondary supplychamber, is connected to a brake-fluid tank (not shown) throughconventional means 19.

The master cylinder 10 also comprises two pistons, namely a primarypiston 20 and a secondary piston 22. In a manner known per se, theprimary piston 20 is connected to a brake pedal (not shown) to bedepressed by the driver. And the secondary piston 22 separates theprimary pressure chamber 14 from the secondary pressure chamber 16.

Both pistons 20, 22 bear conventional primary and secondary valves, 24and 26 respectively.

The master cylinder 10 also comprises conventional means 28 for thehydraulic connection of the primary pressure chamber with conventionalbraking-force simulation means 30.

The hydraulic connection means 28 are closable using a O-ring 32, borneby the secondary piston 22 and intended for a cooperation with anannular bearing seat 34 provided in the body 12. The O-ring 32 and thebearing seat 34 are the respective complementary moving and stationarysealing means for the hydraulic connection means 28.

The secondary piston 22 is axially movable, parallel to the axis Xrepresented in FIG. 1, between a rest position, in which the O-ring 32and the bearing seat 34 are spaced apart from each other, and a positionin which the O-ring 32 and the bearing seat 34 are cooperating with eachother, with the result that the connection means 28 are closed.

The secondary piston 22 comprises two portions, i.e. a proximal portion22P and a distal portion 22D, capable of an axial motion in relation toeach other in order to adjust the relative axial position of the O-ring32 and of the bearing seat 34 when the secondary piston 22 is in itsrest position, as shown in FIG. 1.

The proximal portion 22P of the secondary piston comprises amale-threaded element M for a screwed cooperation with a female-threadedelement F provided in the distal portion 22D of the secondary piston.The male element M constitutes a screw for a cooperation with the femaleelement F, which forms a nut, so as to connect the proximal portion 22Pwith the distal portion 22D of the secondary piston.

A check nut 36 is intended to be screwed on the male element M, whileresting on the female element F, so as to lock these elements M and F ina predetermined relative position.

In a preferred manner, the proximal portion 22P of the secondary pistoncomprises a transverse slot 38 intended for a conventional tool for themanoeuvring of such proximal portion.

The proximal portion 22P of the secondary piston bears the O-ring 32.The distal portion 22D of the secondary piston is intended for acooperation with a pin 40, which is rigidly locked with the body 12 andwhich forms a rest stop, defining the rest position of the secondarypiston 22.

The pin 40 extends substantially transversely to the travel direction ofthe secondary piston 22 and is accommodated in an elongate cavity 42provided in the distal portion 22D of said piston.

The elongate cavity 42 communicates with the secondary supply chamber18. The latter is capable of being connected with the secondary pressurechamber 16 through a passage 44, closable by means of the secondaryvalve 26.

The secondary valve 26 is fitted with a control stem 46, extendingthrough the passage 44 and intended for a cooperation with the pin 40 soas to keep the valve 26 in the open position, when the secondary piston22 is in its rest position, as shown in FIG. 1.

Quite conventionally, the secondary piston 22 is resiliently returned toits rest position by means of a spring 48, called the secondary spring.

As regards the adjustment of the stroke of the secondary piston 22between its rest position, as shown in FIG. 1, and the position in whichthe connection means 28 are closed (cooperation of the O-ring 32 withthe bearing seat 34), such stroke being called the dead stroke C, thefollowing procedure may be carried out.

Initially, the secondary piston 22 is in its rest position as shown inFIG. 1, and the primary piston 20 has not been fitted yet inside thebody 12 of the master cylinder.

The primary pressure chamber 14 is supplied with pressure gas, moreparticularly air, and a pressure sensor is arranged in the braking-forcesimulation means 30. A travel sensor is provided for the measurement ofthe travels of the secondary piston 22.

The connection means 28 being in the open state, the pressure air,contained inside the primary pressure chamber 14, flows into thebraking-force simulation means 30.

Then, the secondary piston 22 is moved against the resilient returningforce of the spring 48 until the connection means 28 are closed throughthe cooperation of the O-ring 32 with the bearing seat 34. Such sealingis detected by the pressure sensor because a pressure change appears inthe braking-force simulation means 30.

The dead stroke C is determined using the travel sensor for thesecondary piston 22.

Therefore, the value of the measured dead stroke C has only to becompared with the desired one and, if these values do not match, thedead stroke C is brought to the desired value by either screwing orunscrewing the proximal portion 22P of the secondary piston relative tothe distal portion 22D of said piston.

Once the adjustment has been carried out, both parts 22P and 22D of thesecondary piston are locked in position in relation to each other by thetightening of the check nut 26, after the secondary piston 22 has beenremoved from the body 12.

FIG. 2 shows a tandem master cylinder for an electrohydraulic brakingsystem according to a second embodiment of the invention. In FIG. 2, thesame reference numerals designate the same elements as in FIG. 1.

In this second embodiment of the invention, the proximal portion 22P ofthe secondary piston exhibits a generally tubular shape, closed at itsdistal end.

Therefore, the proximal portion 22P of the secondary piston defines aninner cavity for the insertion of a tool intended for the manoeuvringand/or the radial deformation of this proximal portion 22P.

Thus, once the dead stroke C has been adjusted through either thescrewing or the unscrewing of the proximal portion 22P of the secondarypiston relative to the distal portion 22D of said piston, these portions22P and 22D are locked in position in relation to each other by theradial deformation of at least one of the threaded surfaces of the maleand female elements, M and F respectively.

FIG. 2 illustrates inner dents 50 made in the proximal portion 22P ofthe piston, using a conventional tool so as to obtain a radialdeformation of at least one of the threaded surfaces of the male andfemale elements, M and F respectively.

In this second embodiment of the invention, the secondary piston 22 doesnot need to be removed from the body 12 so as to lock both parts 22P and22D in position in relation to each other.

Among the advantages afforded by this invention, it should be noted thatthe dead stroke of the secondary piston 22 can be easily adjusted andthat, therefore, it is possible to limit the quantity of brake fluid,which is transferred from the primary pressure chamber 14 to thebraking-force simulation means 30 when the driver depresses the brakepedal, as the braking system (including the master cylinder 10 accordingto the invention) is in the emergency operation configuration.

1. A master cylinder intended for an electrohydraulic braking system, ofthe type comprising: a body (12), defining a primary pressure chamber(14) and a secondary pressure chamber (16), separated by an axiallymovable piston (22), the so-called secondary piston; means (28) for thehydraulic connection of the primary pressure chamber (14) withbraking-force simulation means (30); and complementary moving andstationary means (32, 34) for the sealing of the hydraulic connectionmeans (28), and borne by the secondary piston (22) and the body (12)respectively, the secondary piston (22) being axially movable between arest position, in which the moving and stationary sealing means (32, 34)are spaced apart from each other, and a position in which said movingand stationary sealing means (32, 34) are cooperating with each other,characterised in that the secondary piston (22) comprises two portions(22P, 22D), capable of an axial motion in relation to each other inorder to adjust the relative axial position of the moving and stationarysealing means (32, 34) when the secondary piston (22) is in its restposition.
 2. The master cylinder according to claim 1, characterised inthat said portions (22P, 22D) of the secondary piston comprise a maleelement (M) and a female element (F) respectively, connecting up theseportions of the secondary piston and provided with complementarythreaded surfaces, in such a way that the male element (M) constitutes ascrew and the female element (F) forms a nut.
 3. The master cylinderaccording to claim 2, characterised in that it comprises means (36; 50)for the locking of the male element (M) and of the female element (F) ina predetermined relative position.
 4. The master cylinder according toclaim 3, characterised in that the locking means comprise a check nut(36), screwed on the male element (M) and resting on the female element(F).
 5. The master cylinder according to claim 3, characterised in thatthe locking means comprise at least a radial deformation (50) of one ofthe threaded surfaces of the male element (M) and of the female element(F).
 6. The master cylinder according to claim 5, characterised in thatthe stationary sealing means comprise an annular bearing seat (34)provided in the body, and in that the moving sealing means comprise aO-ring (32), borne by the secondary piston (22) and intended for acooperation with said bearing seat (34).
 7. The master cylinderaccording to claim 6, characterised in that a first portion (22D) of thesecondary piston is intended for a cooperation with a rest stop (40),rigidly locked with the body, while the second portion (22P) of thesecondary piston bears the moving sealing means (32).
 8. The mastercylinder according to claim 7, characterised in that the rest stopcomprises a pin (40), which is rigidly locked with the body (12) andwhich extends substantially transversely to the travel direction of thesecondary piston (22), such pin being accommodated in an elongate cavity(42) provided in the first portion (22D) of said piston.
 9. The mastercylinder according to claim 8, characterised in that the elongate cavity(42) communicates with a brake-fluid supply chamber (18), called thesecondary supply chamber, and in that the first portion (22D) of thesecondary piston is provided with a passage (44) connecting up thesecondary supply chamber (18) and the secondary pressure chamber (16),and closable by a valve (26).
 10. The master cylinder according to claim9, characterised in that the valve (26) is fitted with a control stem(46), extending through the passage (44) provided in the secondarypiston, and intended for a cooperation with the pin (40) so as to keepthe valve (26) in the open position.